This invention relates to a method to operate a high-voltage d.c. (HVDC) transmission line between two a.c. networks during normal and malfunctioning conditions and an apparatus for said purpose.
In this apparatus a first converter is connected to a first a.c. network, NA, and normally operating as a rectifier, feeds d.c. current into the HVDC transmission line at the first station, A, while a second converter, mounted at a second station, B, and feeding into a second a.c. network, NB controls the voltage of the HVDC transmission line. Any modification of the HVDC transmission line voltage determined by the second station effects a change of the current fed into the first station after a lag time determined by the transmission time or travel time of the HVDC transmission line, as long as the control angle of the first converter is not reset. Conversely, any change of the d.c. current supplied from the station, A, effects a change of the commutation time of the thyristors of the second converter, whose control angle has to be monitored and, if necessary, shifted accordingly in order to maintain the inverter step limit.
For that reason, the two stations are generally connected by remote control lines and interact, e.g., by a so-called marginal current signal, to form the control angle. The controllers installed in the stations are set for relatively slow action in order to allow time for controller interaction and for settling time.
If a voltage short circuit occurs in the station, A, then shortly thereafter the d.c. current in the HVDC transmission line lapses or the second converter is shut down in order to avoid further power supply into the short circuit a, e.g., rectifier malfunction. The d.c. connections of the first converter can be short-circuited via a short-circuit switch referred to as a bypass circuit in order to detour any existing d.c. current by the converter and maintain the current in the line at a reduced voltage.
This short-circuit switch can be designed either as a relatively slow-acting mechanical switch or as a thyristor appropriately rated for relatively low currents. When installed, said bypass-thyristor is not suited to carry current supplied by the station, B, in case said station has not been shut down, but instead is switched over from inverter operation and is operated as a rectifier during the malfunction. Such a bypass current would mean a reactive current for the a.c. system, NB, because the HVDC transmission line becomes a long current charged inductor and could involve high levels, e.g., 60 percent, of the nominal current.
While the converter thyristors available in any event for normal operation of the first converter could carry such a current, they could only be fired using expensive firing devices if due to the rectifier malfunction the HVDC transmission line had gone dead and if also the malfunctioning system, NA, could not provide firing energy.
For that reason said bypass operation in which converter A is bridged after a rectifier malfunction and converter B is operated as a rectifier has not, heretofore, been considered a reasonable approach simply for cost reasons.
Furthermore, the control mechanisms of the station, B, are set for relatively slow action for the reasons explained above, and said bypass operation would only result in a reactive load for the system, NB, which would be difficult to control.
In case of a malfunction in the station, B, similar conditions would prevail. During every voltage short circuit the HVDC transmission line current would flow into this short circuit, and the inverter would fail. It is shut down, and the HVDC transmission line discharges via the inverter thyristors until the current goes out. A short-circuit switch also installed at that point can be closed until voltage resumes, and during this malfunction rectifier A is shut down. If the converter thyristors themselves were used as short-circuit switches, i.e., the bypass circuit, then additional measures for correct selection of the firing pulse would be required to fire the thyristors in accordance with the system cycle when voltage resumes.
A further complicating factor is that the remote signal lines have transmission and processing time factors of their own so that the signals required to operate the functioning station regarding the respective status of the other station are only available after substantial delays.
For that reason heretofore normal operation of the HVDC transmission line has been mostly designed in such a manner that, for example, a failure of the inverter or other malfunctions were avoided, if at all possible; however, when a malfunction occurred, the HVDC transmission line would be held without current by shutting down both converters.
An object of this invention is thus to define a mode of operation of the HVDC transmission line whereby the HVDC transmission line can be utilized even in the event of a malfunction in one of the stations.
It is another object of this invention to provide a method for operating a HVDC transmission line between two a.c. networks continuously during normal and malfunctioning conditions.
It is a further object of this invention to provide a control method responsive enough for operation during malfunctioning conditions.